Fuel consumption changes on 2018 GMC Sierra 3500
I have a 2018 GMC Sierra 3500 HD with the 6.6 litre diesel engine and Allison transmission. The truck generally gets quite respectable mileage, using about 10-11 litres of diesel per 100 kilometres (22-23 MPG) on the highway. From time to time, though, the fuel consumption will abruptly increase from this to 16-17 litres per 100 kilometres (14-15 MPG) for about 20 minutes then return to normal. This is on level roads at the same steady speed and wind conditions. Fuel consumption will even remain at 4-5 litres per 100 kilometres (52 MPG) when coasting downhill where it normally drops to near zero. Why does this occur? – Many thanks, Len
I believe what you are noticing is the truck is performing an active regeneration procedure, which involves the spraying of additional fuel into the exhaust at certain times to create significant heat so as to burn off accumulated soot (particulatematter) in the diesel particulate filter (DPF).
Duramax and other diesel engines built since 2011 employ a sophisticated and sometimes problematic exhaust emissions treatment system including (from front to rear on Duramax): hydrocarbon injector (HCI), diesel oxidation catalyst (DOC), diesel exhaust fluid (DEF) injector, selective catalyst reduction (SCR) catalyst, and diesel particulate filter (DPF). The parts we’ll focus on are the 9th injector (HCI) and the DPF.
Your diesel particulate filter traps soot and requires periodic cleaning (oxidation) on a regular basis. A combination of heat added due to fuel being injected (about 1000-1300 degrees F / 538-704 degrees C) and preciousmetals within the DPF create this reaction. If the truck is driven on the highway frequently, some soot cleaning occurs naturally (passive regeneration). Active regeneration occurs when the system either senses a level of differential pressure before and after the DPF indicating about 42 grams of soot has built up or 700 miles have accrued since the last active regeneration process. Raw fuel is injected into the exhaust via the HCI to raise exhaust temperature for a period of approximately 20minutes, as you have noticed on your fuel consumption readout. Other clues active regeneration is occurring are a different exhaust sound, elevated idle speed, an odor, and more noticeable heat beneath the truck as it is stationary. Here are three informative articles that help explain this process:
https://blog.duramaxtuner.com/blog/how-does-regen-work-in-anlml-duramax
https://www.knowyourparts.com/technical-resources/diesel-engines/ diesel-particulate-filter/
http://www.duramaxhub.com/duramax-emissions.html
How do rain sensing wipers work? I love mine – it’s like magic! – Susan G.
Most rain-sensing wiper systems employ a rain sensor, located on the inside of the windshield behind the rear view mirror. Infrared light is projected outward through the windshield glass at a 45 degree angle. Depending on how much light reflects back to the sensor (rain, snow, or dirt scatters the light, reducing reflect-back), the wipers are turned on at an appropriate speed and delay frequency Vehicle speed is often another input for automatic wiper operation.